Materials, wherein a charge transport molecule which serves as a charge transport site are dissolved or dispersed in a matrix material, such as a polycarbonate resin, or materials, wherein a charge transport molecule structure pendent as a pendant from a polymer backbone, such as polyvinyl carbazole, are known in the art. These materials have been extensively used as materials for photoconductors in copying machines, printers and the like.
For the above conventional charge transport materials, in the case of dispersive charge transport materials, that the charge transport molecule has high solubility in the polymer as a matrix is preferred from the viewpoint of improving the charge transport capability. In fact, however, bringing the charge transport molecule to a high concentration in the matrix leads to crystallization of the charge transport molecule in the matrix, and, for this reason, the upper limit of the concentration of the charge transport molecule is generally 20 to 50% by weight although it varies depending upon the kind of the charge transport molecule. This means that the matrix not having charge transport capability occupies not less than 50% by weight of the whole material. This in turn raises a new problem that the charge transport capability and response speed of a film formed from the material are limited by the excess matrix present in the material.
On the other hand, in the case of the pendant type charge transport polymer, the proportion of the pendant having charge transport capability can be increased. This polymer, however, involves many practical problems associated with mechanical strength, environmental stability and durability of the formed film, film-forming properties and the like. In this type of charge transport material, the charge transport pendants are locally located in close proximity, and the local proximity portion serves as a stable site in hopping of charges and functions as a kind of trap, unfavorably resulting in lowered charge mobility.
For all the above charge transport materials, electrical properties of such amorphous materials raise a problem that, unlike crystalline materials, the hopping site fluctuates in terms of space, as well as in terms of energy. For this reason, the charge transport depends greatly upon the concentration of the charge transport site, and the mobility is generally about 1.times.10.sup.-6 to 1.times.10.sup.-5 cm.sup.2 /v.s which is much smaller than that of molecular crystal, 0.1 to 1 cm.sup.2 /v.s. Further, the amorphous materials have an additional problem that the charge transport properties depend greatly upon temperature and field strength. This is greatly different from the crystalline charge transport materials.
A polycrystalline charge transport material is a promising material in applications where a charge transport layer having a large area is necessary, because it can form an even charge transport film having a large area. The polycrystalline material, however, is inherently uneven from the microscopic viewpoint and involves a problem that a defect formed in the interface of particles should be inhibited.
Accordingly, the present invention aims to solve the above problems of the prior art and to provide a novel charge transport material which simultaneously realizes advantages of the amorphous materials, that is, structural flexibility and evenness in a large area, and advantages of the crystalline materials having molecular alignment and is excellent in high-quality charge transport capability, thin film-forming properties, various types of durability and the like.
According to the material of the present invention, the anisotropy of the charge mobility derived from the molecular alignment can be expected and is structurally flexible, permitting the alignment to be regulated by external stimulation. Materials useful as the charge transport material are not less than 1.times.10-5 cm.sup.2 /v.s in terms of the carrier mobility. When the carrier mobility is less than 1.times.10.sup.-5 cm.sup.2 /v.s, no high-speed response can be expected.